The present inventions relate to improvements methods and apparatus used in the production of hydrocarbons from subterranean wells completed with gravel packing. More particularly the present inventions relate to improvements in completing new and reworking existing gravel packed wells wherein fracturing is conducted with the gravel pack installed in the wellbore.
As a result of drilling and completion practices, the formation permeability near the wellbore can be reduced. Drilling fluid invasion of the formation, dispersion of clay, presence of mudcake, and cement tend to reduce the formation permeability around the wellbore. Perforation, hydraulic fracturing and other completion practices tend to increase the effective permeability around the wellbore. The zone of reduced formation permeability is called a xe2x80x9cskinxe2x80x9d and the resulting effect on well performance is called the xe2x80x9cskin factor.xe2x80x9d xe2x80x9cSkin Factorxe2x80x9d is described and defined in PETROLEUM WELL CONSTRUCTION, by Economides, et al 1998, pp 8-10 and 405-409 incorporated herein by reference. The xe2x80x9cskin factorxe2x80x9d can be used to indicate the well performance. The factor is positive for a damaged well and negative for a stimulated well. The factor indicates how the area around the well compares to the original reservoir permeability.
Oil and gas wells are often completed in unconsolidated formations containing loose and incompetent fines and sand, which migrate with fluids produced by the wells. The presence of formation fines and sand in the produced fluids is disadvantageous and undesirable in that the particles abrade pumping and other producing equipment, raise the wells skin factor and reduce the fluid production capabilities of the producing zones in the wells. It is common to install gravel packs of particulate material in the well to serve as filters to keep fines and formation sand from flowing back with produced fluids into the wellbore. The size of the sand in the gravel pack is selected such that it prevents formation fines and sand from flowing into the wellbore with produced fluids.
Subterranean zones or formations have been stimulated by creating fractures in the zones and depositing particulate material commonly called proppant in the fractures to maintain them in open positions. In cased wells fracturing is conducted after perforation of the casing and formation. In some formations the fracturing process can itself create fines in the fractured formation. Typically gravel packing occurs after perforation and fracturing. However, fractures and perforations open flow paths into the formation that create problems in process and equipment used for forming gravel packs.
Downhole particulate control apparatus have been used in gravel packed wells to restrict or prevent the return of particulate into the wellbore. Sand screens, liners and the like have commonly been installed in the wellbores penetrating unconsolidated zones. In a typical gravel pack completion, a screen is placed in the wellbore inside the gravel pack. The term xe2x80x9cscreenxe2x80x9d is used in its broadest functional sense meant to mean structures that screen out the particulate and includes porous structures that pass well fluids but prevent or block the passage of particulate. Screens include not only the classic wire woven varieties but also other structures including porous solids, slotted, perforated or silted liners and the like. The screen is sized to prevent the particulate in the slurry from flowing therethrough. The screen is typically connected to a tool that includes a production packer and a cross-over tool. The cross-over tool is in turn connected to a work or production string.
Particulate material which is usually graded sand, often, referred to in the art as gravel, is pumped in liquid slurry down the work or production string and through the cross over whereby it flows into the annulus between the screen and the wellbore. The particulate is deposited in the annulus around the screen whereby it forms a gravel pack. In cased well the wellbore is defined by the inside of the casing, in open hole completions the wellbore is defined by the inside of drilled hole. In forming the gravel packs the liquid leaks off into the formation and into the screen leaving the gravel in place. In a conventional well where fracturing (and possibility perforation) have preceded gravel packing procedures, liquid forming the slurry can leak off uncontrollably into the subterranean zone causing defects in the gravel pack. In cased wells the leak off is through the perforations in the casing. In open hole wells the leak off is through cracks, holes, and fractures in the formation sealing mud cake. Liquid leak off through the screen can be controlled with flow pipes and the like but uncontrolled liquid leak off into the formation remains a problem.
Problems encountered in forming gravel packs or in placing proppant in the fractures can raise the skin factor and negatively effect well production. These problems and procedures associated with gravel packing are described in U.S. Pat. No. 5,934,376 (the entire disclosure of which is incorporated by reference). For example, a problem often encountered in forming gravel packs, particularly gravel packs in long and/or deviated unconsolidated producing intervals, is the formation of sand bridges in the annulus. That is, non-uniform sand packing of the annulus between the screen and the wellbore often occurs as a result of the loss of carrier liquid from the particulate slurry into high permeability portions of the subterranean zone which in turn causes the formation of sand bridges in the annulus before all the particulate has been placed. Frac packing and perforation are designed to create zones of high permeability, which in turn have the disadvantage of contributing to the formation of sand bridges. The sand bridges block further flow of the slurry through the annulus, which leaves voids in the annulus. When the well is placed on production, the flow of produced fluids is concentrated through the voids in the gravel pack, which soon cause the screen to be eroded resulting in the migration of fines and sand with the produced fluids to result.
In addition, mechanical chokes or pinching can form when the fractures close at the voids as proppant is over displaced away from the wellbore during the flush stage. In many cases many of these existing well production inhibiting problems are not recognized until after the well is completed and in service. Reworking the well to eliminate these problems is uneconomic because of the cost of removing the existing down hole particulate control apparatus. In addition, some previously properly gravel or frac packed wells could have improved production by performing formation perforation followed by fracturing. However, the installed down hole particulate control apparatus cannot be economically removed.
Thus, there are needs for improved methods and apparatus for economically completing and reworking gravel packed wells to stimulate the producing interval in subterranean zones with existing particulate control apparatus in place.
The present inventions contemplate an improved method and apparatus for completing and reworking wells by fracturing and perforation with existing gravel packs and downhole production equipment in place. This allows the well to be completed or reworked to stimulate and in turn improve production without incurring the expense removing the existing gravel packs and screen. In addition, gravel packing in new wells can be completed prior to perforation and frac packing, thereby reducing problems in gravel packing associated with leak off into the formation.
The present inventions contemplate perforating and frac packing wells after gravel packing. The present inventions are applicable to reworking existing wells and completing new wells. By gravel packing before perforating and fracturing, fluid leak off into the formation through the perforations during gravel packing is prevented. The quality of the gravel pack can be controlled and improved. In existing wells additional perforations and fracture pack procedures can be performed to rework the wells.
In another aspect of the present inventions perforation and frac packing is performed through the in place screen. A consolidating particulate is used in at least the final portion of the frac packing process and is allowed to accumulate and consolidate in and adjacent the wellbore. Thereafter, the particulate in the wellbore is removed while the consolidated particulate adjacent the wellbore prevents flow back of particulate. Thereafter, a particulate control apparatus such as a radially expandable screen can be installed inside the in place screen.
The novel features of the inventions are set forth with particularity in the claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.